This invention relates to a flat type fluorescent display tube capable of displaying a plurality of digits, letters or symbols.
As is well known in the art, a prior art fluorescent tube adapted to display digits of a number of orders of magnitude comprises an insulating substrate formed with a plurality of digit display units, a plurality of independent digit selection grids mounted on the insulating substrate to respectively oppose the digits, and a cathode electrode for emitting thermal electrons. At least the display units, the grids and the cathode electrode are sealed in a vacuum envelope. Such prior art tube is disclosed in U.S. Pat. No. 3,723,789 to Tanji. Each digit display unit is constituted by a plurality of segment electrodes, and phosphor films coated on the segment electrodes. Corresponding segments of the digit display units are commonly connected to external terminals so as to display a desired digit of a desired order by using so-called dynamic drive and by designating a digit selecting grid.
In the fluorescent display tube of the dynamic driving type described above, since the digit display units are arranged in a plane on the substrate, where it is desired to increase the number of digits, there arises the following problems.
Firstly, if one wishes to increase the number of displayable digits without increasing the size of the substrate, it is necessary to decrease the size of the digit display units and to decrease the spacing between adjacent display units. However, small display units are difficult to see. Moreover it is impossible to decrease the spacing between the digit display units beyond a certain limit from the standpoints of electric insulation and construction.
It is also possible to increase the size of the substrate. This not only increases the size of the tube but also results in the change of the layout of a device into which the display tube is to be incorporated.